Sapphire is a generic term for alumina (Al2O3) single-crystal materials. Sapphire is a particularly useful material for use as windows for infrared and microwave systems, optical transmission windows for ultraviolet to near infrared light, light emitting diodes, ruby lasers, laser diodes, support materials for microelectronic integrated circuit applications and growth of superconducting compounds and gallium nitride, and the like. Sapphire has excellent chemical stability, optical transparency and desirable mechanical properties, such as chip resistance, durability, scratch resistance, radiation resistance, a good match for the coefficient of thermal expansion of gallium arsenide, and flexural strength at elevated temperatures.
Sapphire wafers are commonly cut along a number of crystallographic axes, such as the C-plane (0001 orientation, also called the 0-degree plane or the basal plane), the A-plane (1120 orientation, also referred to as 90 degree sapphire) and the R-plane (1102 orientation, 57.6 degrees from the C-plane). R-plane sapphire, which is particularly suitable for silicon-on-sapphire materials used in semiconductor, microwave and pressure transducer applications, is more resistant to polishing than C-plane sapphire, which is typically used in optical systems, infrared detectors, and growth of gallium nitride for light-emitting diode applications.
The polishing and cutting of sapphire wafers can be an extremely slow and laborious process. Often, aggressive abrasives, such as diamond, must be used to achieve acceptable polishing rates. Such aggressive abrasive materials can impart serious surface damage and contamination to the wafer surface. Typical sapphire polishing involves continuously applying a slurry of abrasive to the surface of the sapphire wafer to be polished, and simultaneously polishing the resulting abrasive-coated surface with a rotating polishing pad, which is moved across the surface of the wafer, and held against the wafer surface by a constant down-force, typically in the range of about 5 to 20 pounds per square inch (psi).
It has been hypothesized that the interaction of sapphire and colloidal silica under the temperature and pressure of polishing pads leads to an energetically favorable chemical reaction for the formation of aluminum silicate dihydrate species (i.e., Al2O3+2SiO2→Al2Si2O7.2H2O). The hardness of these various hydrates and aluminum species are assumed to be lower than the underlying sapphire, resulting in a slight film, which can be easily removed by colloidal silica slurries without damaging the underlying surfaces. Prior practices have also focused on increasing polishing temperatures to increase the rate of alumina hydrate film formation and thus the removal rate. It has also been shown that increasing salt concentrations in basic colloidal silica slurries have increased removal rates for both c and m plane sapphire. Finally adding aluminum chelating agents, such as EDTA derivatives and ether-alcohol surfactants enhances polishing performance by tying up and lifting off the surface alumina species and suspending the slurry components for a cleaner wafer surface.
None of these developments in sapphire polishing however have completely resolved polishing performance due to the typically slow polishing rates achievable with other abrasive materials. Accordingly, there is an ongoing need for compositions, kits and methods to enhance the efficiency of polishing of sapphire surfaces.